Although the Wright Brothers are credited with the first powered flight, even ancient man's dreams and ambitions have shown a fascination with the thought of leaving the ground and defying gravity. Logic would dictate the idea of flight was there in ancient times, I would think from observation of birds. What was missing for ancient man was a source of propulsion and materials strong and light enough. The idea of air passing over a flat surface, edge on, would also have been obvious from the observation of birds, leaves, and the wind raising heavy flat objects off the ground.
From the time of Leonardo da Vinci, man's paradigm of a flying machine involved powered rotation of masses, such as a shaft with fan blades on the end for the purpose of moving and compressing air. Rotating masses create torques and stress loading that requires airframes to be robust and consequently heavier than one would like.
The common methods of aircraft and missile propulsion today are propellers, gas turbine (jet engines) rockets, ram and scram jets. Propeller driven aircraft use volatile liquid aviation fuel to power a reciprocating or gas turbine engine, which in turn drives a propeller, or rotors in the case of a helicopter. The propeller or rotor pulls sufficient air through its diameter to push the aircraft through the air at sufficient speed for the wings to provide lift or in the case of a helicopter, rotors to provide downward thrust for vertical lift. These conventional types of propulsion are heavy, noisy, complex, expensive, and subject to mechanical failure because of so many moving parts. Vibration and torque are also of major consideration in the design of aircraft because of the requirements for stronger and consequently heavier airframes to handle the stresses produced by these torque and vibration forces. Even the engines themselves require high precision parts and because of torque's and loads, strong and heavy metals are required in the structure.
In the case of the reciprocating engine, air and liquid fuel are inducted into a closed cylinder through a set of mechanical valves, in appropriate ratios. This mixture is compressed by a piston connected to a crankshaft. As the piston reaches the top of its stroke, a spark is generated, in the combustion chamber, above the piston, igniting the compressed fuel air mixture, forcing the piston down and turning a crank shaft. The crankshaft in this type of engine is connected to a gearbox through which the torque is taken from it, geared up or down and delivered to the propeller or rotor system. Gearboxes, shafts and other stressed members have to be made of hardened steel because of the torque loads placed on them, this adds to the weight problem. These type engines are plagued with drawbacks, from weight, vibration, icing problems, dust choking, thermal inefficiency, and cost of maintenance, which over the life of the aircraft is three to one, maintenance to flight hours.
Existing jet engines use their liquid fuel to heat and expand air through a heavy and costly precision turbine wheel. This turbine wheel rotates at 30,000 RPM and drives a shaft that is coincident with the longitudinal axis and extends to the intake of the engine. Very precise, expensive, and fragile multistage compressor blades are attached to this shaft and angled to provide air for the turbine wheel. These engines burn fuel continuously at an enormous rate, much of this fuel is wasted in heat and having to run continuously, because once started the ring of fire must be maintained to light off the incoming jet fuel. The whine of the turbine and the massive flow of partially burned JP5 makes a very loud noise and leaves a huge heat signature. Jet and turbine engines are also subject to flameout during power changes, so power changes have to be slow, another disadvantage of engines with rotating masses.
Jet and turbine engines, by their very nature, are very vulnerable to foreign object damage (FOD) because of, tight tolerances, fragile turbine blades, and the speed at which they rotate. Even very small objects can be a very serious problem if ingested into a jet engine, dust, volcano ash, birds and any kind of debris can be ingested during landing and takeoff off aircraft. This is especially true in combat zones or hastily prepared airfields, small objects ingested can cause serious damage, failure, or even explosion. The weight and forces caused by the rotating masses in jet and turbine engines requires the aircraft to be sturdy and consequently heavy and rigid. The cost of manufacture and maintenance is another drawback to jet engines. The frontal area on these type engines causes profile drag on the aircraft and produces a large radar cross section.
Solid Fuel Rockets
Conventional rockets are essentially a tube with a precise and concentric restriction known a De Laval nozzle or Venturi. This tube or rocket motor is filled with a volatile energetic fuel that burns on the edge of detonation. Rockets must carry the total mass of fuel and oxidizer on board. Once fired there is no shutoff. The fuel is often toxic and dangerous to handle and to manufacture. These engines are also very complex and expensive to build. They also require exotic materials and tight tolerances in their construction.
Leaks in rockets have serious consequences as seen with the Challenger explosion. Solid fuel rockets are essentially a large stick of explosive that burns at a controlled rate just below detonation. Any changes in the mixture such as air entering could cause detonation of the rocket motor. Detonation occurs when the burn velocity exceeds the sonic velocity of that material. Carrying an oxidizer for a solid fuel rocket on board when one is traveling through an oxygen rich atmosphere can be very dangerous. These types of rockets are very noisy and must have an area clear of personnel when launched.
Liquid Fuel Rockets
Robert H Goddard is credited with the first launch of a liquid fueled rocket in 1926. Pedro Paulet, a Peruvian, also claimed he had conducted experiments on liquid rockets in the 19th century, while a student in Paris. Liquid fueled rockets carry the fuel and oxidizer in separate tanks and require pumps and delivery lines to combine the fuel and oxidizer in a combustion chamber. Liquid rockets are desirable because of their energy density to containment mass required such as tanks, pumps and injectors.
In the case of liquid rockets the fuel and oxidizer are delivered through tubing lines from containment tanks by conventional or turbo pumps to a combustion chamber. Some of these pumps mix the fuel and oxidizer by spinning it into a vortex and causing mixing by centrifugal forces. Other methods of mixing are converging nozzles that collide the ingredients in a tight stream causing them to atomize, making for a more easily ignited mixture and a more complete burn.
There are many drawbacks to liquid rockets, tanks, pumps, injectors, and delivery lines are subject to cryogenic temperatures which can contribute to collapse of the tanks due to vacuum created inside the tank when empty. This along with the potential for instability of the projectile due to motion of the liquid in the tanks and icing adding weight makes liquid rockets far from an ideal propulsion system. Pumps and injectors exposed to −253 deg C., the storage temperatures of oxygen and hydrogen, commonly used combinations of fuel and oxidizer, have to be heated by hot gas circulation systems, stealing power from the thrust and adding weight and complexity.
The ignition methods for liquid rockets are not completely safe. Hard starts are common and cause explosions which can propel debris many meters at high velocity creating deadly hazards to personnel and property. This condition is precipitated when the ignition system fails to ignite the fuel mixture at the correct time and intensity. The fuel mixture builds up in the combustion chamber, when the igniter finally lights there is an explosion. Depending on how rich the fuel mixture and the blast pressure rating of the housing, this explosion can throw fragments many meters at high velocity. Such explosions can rupture the tanks and lead to the potential of a fuel air explosion.
The most used method of ignition involves flame, such as from a sparkplug, hot bridge wires, and many other similar means are also used. These engines can be throttled unlike solid fuel rockets. Although mono propellants are mostly used because of complexity and the hazardous nature of the ingredients. The volatility of some possible mixes make the necessity of a reliable ignition source paramount, many other mixes of propellants are possible with a reliable and sustainable ignition source.
Scram and ram jets are another type of propulsion system with their own set of problems. These types of engines cannot operate at zero airspeed and need to be taken aloft or have another power source and propelled to about 350 mph before there is enough air flow to prevent back pressure when the fuel is applied. Ram jets slow the air and compress it to subsonic velocity before combustion, whereas Scram jets operate at supersonic air flow throughout their operational range. There are many high tech problems with this type of engine such as the burn time of the fuel, if too slow it gets swept away before full combustion. Another severe problem that there does not seem to be a solution for at present, are supersonic shock waves which interrupt the smooth flow of air, disturb the fuel distribution and create temperatures beyond control. Because of this temperature problem supersonic powered flight time must be limited.